The present invention relates to a fluid seal and more particularly to a fluid-tight seal for a cryogenically cooled enclosure.
The problem of sealing a joint between two interconnecting pieces that is subsequently cooled to a cryogenic temperature has been previously recognized. In part, this is the problem addressed in U.S. Pat. No. 3,630,533 by Butler et al., issued Dec. 28, 1971, entitled DYNAMIC SEAL FOR CRYOGENIC FLUIDS. The other problem addressed in Butler et al. is the high temperature sealing problem, which did have an influence on the overall design approach taken. The approach of Butler et al. uses a circular sealing ring made from a fluorocarbon plastic material to seal the two metal tubular couplings. The sealing ring is pressed onto a radially outward surface of one of the metal couplings. The sealing ring has a radially inward protruding annular rib which elastically and inelastically deforms (i.e. cold flows) as the sealing ring is pressed into place. This arrangement effects a sealing engagement between the sealing ring and the metal coupling at temperatures within the range of 70.degree. F. to -423.degree. F. At normal temperatures the inner surface of the sealing ring is held in sealing engagement by the elastic preload induced by the initial interference press fit and deformation of the protruding annular rib. In addition to the pre-load, a circumferential tension is generated in the sealing ring as the temperature decreases because its coefficient of thermal expansion and contraction is greater than the coefficient of thermal expansion and contraction of the metal coupling. Thus, because of the differences in the expansion coefficients of the sealing ring and the metal coupling that it is pressed onto, the sealing engagement between these two members of two different materials becomes tighter as the temperature decreases.
With such a design, each time two metal tubular couplings are to be sealed together, a complex plastic sealing ring must be inelastically deformed into a particular configuration. Since the sealing ring is irreparably deformed by its installation, after separation of the two couplings for maintenance or reasons, it would be necessary to replace the sealing ring before the two couplings could be rejoined. Such a "use once and throw away" approach is wasteful, expensive, and troublesome if the replacement ring is not readily available. A coupling between two members of similar material, which can be joined and separated without requiring a new sealing ring each time is needed. Moreover, if the two members to be joined were made of a fluorocarbon polymer instead of metal, an often advantageous arrangement in electronic equipment, a fluorocarbon polymer sealing ring would no longer be effective since the mechanism based on dissimilar coefficients of thermal contraction between the sealing ring and the member it envelops would no longer exist.
Another type of seal for operation at cryogenic temperatures is known from U. S. Pat. No. 3,361,430 issued Jan. 2, 1968 to Reid for a TEMPERATURE COMPENSATING SEAL. The temperature compensating seal known from Reid is assembled from a number of complex force components. This seal is circular and has an axially open, U-shaped body with a flexible sealing portion. A circular wound spring is located in the U-shaped body and it holds the flexible portion against the surface to be sealed. This spring holds the flexible sealing portion against the surface to be sealed at normal to cool temperatures. However, as the spring is cooled to cryogenic temperatures, the circular wound spring contracts and begins to lose its ability to hold the sealing portion in sealing engagement with the surface to be sealed. At cryogenic temperatures, the circular wound spring is unable to hold the flexible sealing portion of the seal against the surface to be sealed.
For cryogenic temperature operation, a second, temperature responsive ring which circumferentially contracts as the temperature decreases is included in the device of Reid. As the ring contracts, it pushes down on a series of levers spaced around the circumference of the seal. These levers pivot on fulcrums, which are an integral part of the U-shaped seal body, and provide a radially outward force to aid the diminished force of the circular wound spring. Together, at cryogenic temperatures, they sufficiently press the flexible portion of the seal into sealing engagement with the surface to be sealed.
There are a number of problems with this known device. First, it is a fairly complex design for a seal. It requires a U-shaped seal body with a circular wound spring and a series of levers mounted therein. One side of the U-shaped body is extended to receive a temperature responsive ring. The extended side also has a ridge thereon which acts as a fulcrum for each of the levers. Second, these pieces must be carefully assembled to function properly, thereby incurring labor costs. Finally, the design requires a chamber in which the seal can seat. This chamber must be carefully formed in the pieces to be sealed, thereby incurring manufacturing costs.
It is an object of this invention to provide a seal for a plastic cryogenic enclosure.
It is another object of this invention to provide a re-usable seal for a plastic cryogenic enclosure.
It is a further object of this invention to provide a simple and inexpensive seal at cryogenic temperatures for a joint between two plastic pieces.